1. Technical Field
The present invention relates to a compression tuned optical structure; and more particularly, a compression tuned optical structure having force or displacement feedback control.
2. Description of Related Art
There are a host of applications that could exploit the principle of a tunable fiber Bragg grating. These include tunable filters, reconfigurable optical add/drop multiplexers, optical performance monitors, wavelockers, tunable lasers, etc. Each of these applications would benefit from the ability to tune the grating accurately and repeatably and without the need for optical closed loop control, i.e. without needing to measure the wavelength of the grating directly.
In the art, since the wavelength of the Bragg grating is uniquely determined by the strain and the temperature of the grating, in principle, if one could simply measure the strain and the temperature of the grating at all times, then one could always know the wavelength of the grating. In practice, this is accomplished by attaching the grating to an actuator such as a piezoelectric element, then stretching the fiber some determinable amount. If the positional relationship between the actuator and the fiber is maintained, then one can theoretically deduce the Bragg grating wavelength by measuring the displacement of the actuator.
But it is known that if there is some lost motion between the fiber and the actuator, then a measurement of the actuator displacement will result in an erroneous wavelength determination. For example, when strain tuning a coated optical fiber, this effect is almost unavoidable, as the known attachment techniques will involve some sort of epoxy with a limited holding ability. Additionally, tuning the fiber Bragg grating by applying tensile strain is considered to be an unacceptable method from the perspective of fiber reliability, since the lifetime of a fiber can be significantly reduced by continuously stressing it.
Alternatively, another known method encases the Bragg gratings in an all glass element capable of sustaining high compressional loads, which has the potential to be incorporated into a device which can be used to reliably and accurately tune a Bragg grating by strain. The technique was originally applied to pressure transducers and incorporates a glass shell around the device to enable transduction of hydrostatic pressure into compressional strain. The core of the element (the dogbone) can be used in other configurations that allow compressive loads to affect the Bragg wavelength. For example, ends of the glass element can be ground into cone shapes which fit into the cone seats of a body which is mechanically attached to a displacement actuator. This composite glass element Bragg grating has two primary advantages over standard fiber gratings discussed above from the perspective of tunability. The first is that, since the element is placed under compression rather than tension, the device is inherently more reliable. The second is that, because the device can be made of glass with arbitrary dimensions and shapes, the issue of forming a slip-free attachment to an actuator becomes simplified (e.g. glass on metal seats i.e. no epoxy to hold off high forces).
However, if one is concerned with extremely high accuracies, then one cannot ignore the possibility of lost motion or hysteresis even in the glass to metal contact region. For example, over time, the seats may deform slightly, thereby changing the actual displacement of the glass element relative to the actual displacement of the actuator. If the displacement of the actuator rather than the glass element is measured, then there will be an error introduced into the measurement.